Artificial lighting consumes a significant part of all the electrical energy produced worldwide. In homes and offices from 20% to 50% of total energy usage is due to lighting. Today, the main light sources used for general lighting in private households are incandescent lamps (light bulbs and halogen lamps) and compact fluorescent lamps (energy saving lamps). For applications in the commercial sector like offices, shops, restaurants, or hotels, fluorescent lamps (tubular or compact) are the preferred choice due to their advantages in terms of power efficacy and lifetime (total cost of ownership). In addition to these traditional light sources, white LEDs start to enter the market of general lighting. OLEDs are very promising candidates to substitute conventional light sources like incandescent bulbs and fluorescent tubes. OLEDs provide potential for power-efficient large area light sources which will substantially contribute to energy efficient lighting.
To enter the general lighting market OLEDs have to achieve power efficacies of up to 90 lm/W (100 lm/W from OLED100 project) and operational lifetimes of up to 70.000 h (100 000 h from OLED 100 project) (inorganic LEDs). But the OLED technology offers more than potentially efficient and long-living light sources. To pave the road towards applications in general lighting, OLEDs are going to make use of their unique form factors allowing flat light sources covering square meters. OLEDs provide potential for large area light sources that combine revolutionary new lamp properties.
They are thin and flat and at the same time they can be transparent, colour-tuneable or flexible enabling light sources with an unprecedented grade of flexibility in terms of design and application, making them highly appealing for consumers.
The electroluminescent devices are limited by the optical extraction efficiency, the ratio of light generated within the device to light emitted into the ambient. Thus, a significant fraction of the electrically generated light is lost within the device. For an OLED, the optical extraction efficiency can be divided into two components: the efficiency of light coupling from the active layers into the substrate ηOLED-s and the extraction efficiency from the substrate to the ambient ηs-a, i.e., ηex=ηOLED-s*ηs-a.
Light extraction is one key issue, and one common problem is that the efficiency with which light may be extracted is reduced by total internal reflection at interfaces, followed by reabsorption of the reflected light: the light is generated in high index layers (n˜1.8) and this light has to escape to the substrate (generally glass with n˜1.5) and then finally to the air (n=1).